Sinusoids are a natural form of propagation of acoustical and electromagnetic energy. Every sinusoid has an amplitude and an argument, or angle. If the amplitude or the argument of a sinusoid varies with time, then the sinusoid describes motion. If the motion involves propagation, then the sinusoid is called a wave. When the argument varies with time, then descriptions of frequency and phase are commonly used to express the nature and parameters of the motion.
Typically, waves have one or more of its three characteristic values changing with time. Such waves are said to be modulated.
The additive combination of two or more sinusoidal waves results in a single wave which is still sinusoidal. If the frequency of any one of the constituent waves differ from that of any other, then both the amplitude and the frequency of the resultant wave are modulated. Often, the modulation is perceived as a `beat`.
In this description of sinusoidal waves, frequency is defined as the time rate of change of phase. A distinction is needed because there are circuits for measuring frequency and other circuits for measuring phase.
Waves are intentionally modulated to carry message information. In AM radio, the amplitude of a wave is varied in accordance with a message. Messages which are data bit streams are carried by varying the amplitude, frequency, phase, or combinations thereof, of the wave.
Traditionally, regulations and system designs for communication systems prevent any one modulated wave from interfering with any other wave. Techniques for avoiding interference include frequency assignments, geographical placement of transmitters, control of power levels, filtering, etc. The objective of these techniques is to provide a single, usable modulated wave in a defined band of frequencies at any time in a prescribed spatial region. This is done because interfering waves, or carriers, create additional modulation which distorts the message which is recovered by conventional demodulator circuitry.
Despite these efforts, co-channel interference does occur in communication systems. And in some frequency bands, regulations permit many users to simultaneously occupy the same channels so that operation on those channels is then limited by such interference.
One approach to reducing the effects of interference is the use of frequency filters, which are typically electrical or electromechanical. However, filtering can only significantly diminish those frequency components of an interfering signal which is outside a passband of a signal or signals of interest. Moreover, filters often distort the signal of interest within the passband itself. Nonetheless, frequency filtering remains an accepted practice for reducing the effects of noise and other interference.
Another approach to reducing the effects of interference is signal averaging. In averaging, several sample values or records of a voltage signal are obtained. These several signal measurements are then averaged. Unfortunately, the benefits of signal averaging cannot be realized in real time. Moreover, signal at engaging provides only an average value of a signal of interest, which may not be sufficiently accurate to correspond to the actual signal of interest. Lastly, if the mean value of the interference is not zero, and is unknown, then the resolved signal, which is the average value of the signal of interest, is biased by the non-zero mean value of the interference.
Another approach to reducing the effects of interference is to use signal correlation techniques. Correlation is also an averaging process. Consequently, the degree of interference reduction depends on the averaging time allowed for the application. Cancellation of interference in real time is not possible using correlation because correlation requires a priori knowledge of the signal to be effective. Correlation techniques are primarily used in signal detection applications, such as, for example, determining the presence or absence of a signal of a known structure.
Because of the limitations of present communication systems in the presence of interference and because of the limitations of filtering, averaging and correlation techniques for removing in real time the effects of interference on a message of interest, another technique for dealing with interference is desirable.
The inventor of the present invention has U.S. Pat. Nos. 4,859,958 ("patent '958"), 4,992,747 (patent "747"), which are both incorporated by reference as though fully set forth herein. In these patents, a means of demodulation of all of several FM carriers is described. U.S. Pat. No. 5,038,115 ("patent '115"), co-invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In patent '115, phase tracking of input terminal signals is described. In one embodiment of the phase tracking circuit of patent '115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed-forward manner. U.S. patent application Ser. No. 08/214,378 by the inventor of the present invention is also reincorporated by reference as though fully set forth herein. The patent application describes, analytically and geographically, the effect of adding two sinusoids of different frequency.
Prior art which references use of the amplitude and frequency information to remove the effects of interference include the following: A theoretical work by M. A. Bykhovshiy entitled "A Comparison of the Effectiveness of FM Radio Interference Cancellers", Scripta Publishing Co. (Washington, D.C.) ISSN0040-2508/84/0003-130041, 1984, pp. 41-45, considers use of an amplitude detector with a frequency demodulator to remove the effects of very weak interference on a dominant frequency modulated carrier. Theoretical work by Yeheskel Bar-Ness entitled "Adaptive Co-Channel Interference Cancellation and Signal Separation Method", 1989 IEEE Conference Proceedings Ch2655-9/89/0000-0825, considers adaptive means to estimate the amplitudes of the desired and weaker interfering signal. The amplitude information is then used to remove the effect of the interference prior to demodulation. Archibald M. Pettigrew in U.S. Pat. No. 5,341,106 entitled "Amplitude Locked Loop Circuits", granted 23 Aug. 1994, on page 5 describes use of an amplitude-locked loop used in conjunction with a phase-locked loop to decode a frequency modulated signal.
None of the known prior art considers sampling by means of a switch or by means of digital signal processing in conjunction with an envelope detector and a frequency demodulator to remove the effects of interference on a recovered message of interest. The present invention provides novel, simple, robust systems when compared with those prior art systems described herein above.